The invention relates to apparatus and methods for improving the speed and accuracy of biological x-ray radiology such as those used in mammography, lithotripsy, and bone imaging, and to apparatus and techniques for reducing the required x-ray dosage, improving contrast resolution, improving the speed of the process, and improving the speed and convenience of radiopaque needle localization and fine needle aspiration biopsy techniques.
In conventional mammography, a woman sits in front of a mammography screen and places her breast, designated by numeral 3 in FIG. 1, on a support 4 having thereon or therein a screen/film detector 4A that is sensitive to x-rays. A breast compressor plate 2 that is transparent to x-rays presses against the top of breast 3 to flatten it and prevent any movement of it during the mammography process. An x-ray source 1 is turned on to produce x-rays 5. The density of a tumor or microcalcification in the breast is different than that of healthy breast tissue, and may appear as a lighter or darker area on radiographic film 4A. Density variations which may be indicative of lesions appear as variations in darkness of an image on radiographic film 4A.
After the exposure of film 4A to the x-rays exiting from breast 3, film 4A is developed using conventional processes, as indicated in block 6 of FIG. 1. This process typically requires 11/2 to 5 minutes. The film negative obtained from the development process can be used in various ways. The most common way of using the film negative is to simply place it on a light box. The physician then can inspect it and hopefully visually recognize any image density variations that suggest the presence of a tumor or group of microcalcifications. Sometimes a digitizing machine is utilized to scan and completely digitize the image appearing on the film negative and store the digitized image data in a computer. The digitized image usually is used for archival purposes and research purposes. (At the present state of the art, the best image contrast resolution presently available is 8 to 10 bits per pixel.) The computer then can perform various known image processing operations on the data in order to identify various features of the image. Arrow 10 in FIG. 1 designates storage of digitized data, and block 11 represents processing of the image data. The developed film negative typically is stored in a library archive after light box viewing or digitization, as indicated in block 9 of FIG. 1.
There are a number of problems common to the above-described conventional mammography techniques and other biological radiology processes. One problem is that film development processing is much slower than is desirable. Another problem with conventional mammography, lithotripsy, and bone imaging is that x-ray dosages are higher than desirable. The human body should be exposed to as few x-rays as is consistent with the effectiveness of the particular diagnostic x-ray system. Unfortunately, it sometimes is not known until after the step indicated in block 7 and/or 11 of FIG. 1 whether the film is underexposed or overexposed and whether more x-rays of the woman's breast consequently are needed to accurately analyze any possible tumors or microcalcifications. Furthermore, the 8-to-10 bit intensity resolution per pixel contrast which is presently achievable is not as high as desirable.
Another conventional mammography technique utilizes the apparatus and method described above further augmented by a radiopaque needle localization technique. A metal needle is inserted into a particular location of the woman's breast as she stands in front of the mammography screen. The needle is moved to various locations of the woman's breast on a trial-and-error basis to "close in" on the abnormal density locations indicated by the developed films. X-rays are taken and developed for each insertion, until the tip of the needle is located precisely at the site of a possible tumor or group of microcalcifications which have been visually located in accordance with the procedure of block 7 in FIG. 1. Some prior techniques feed the x-ray films to a film digitizer. A computer reads the resulting digitized intensity values and computes how deep the needle tip should be. In a procedure called stereotactic biopsy, two exposures are taken using the same film. The x-ray source is tilted 45 degrees in opposite directions for the two exposures. A computer or an operator measures the displacement of a lesion on the film between the two exposures and from that information computes the depth of the lesion within the breast.
The procedure of obtaining a useable image and moving the needle on a trial-and-error basis to locate it precisely at the site of a likely tumor is time-consuming and very uncomfortable to the woman, who must remain sitting at the mammography screen, without moving, until the needle is properly located. After the needle has been properly located, the patient then may go to surgery for a biopsy, wherein the surgeon follows an incision along a needle to the tip of the needle and removes tissue located thereat for analysis. Alternately, a radiopaque needle biopsy aspiration technique can be performed while the breast remains compressed by plate 2.
It is known in lithotripsy, in which a kidney stone or gallstone is located by ultrasound imaging techniques and high intensity sonic energy is then focused on the kidney stone to shatter it, that it would be desirable to obtain faster determination of the location of the kidney stone or gallstone because the patient is maintained under anesthesia during the procedure. Increase of risk to the patient could be reduced substantially by reducing the time under anesthesia. Similarly, known radiological bone imaging techniques utilize x-rays and CCD (charge-coupled device) cameras, but the amount of time required to obtain images and is greater than desirable, and the amount of x-ray dosage required is greater than desirable.
CCD's have been used in radiological imaging, and are known to have inherent resolution capable of matching or exceeding the resolution of developed x-ray film. See "HIGH RESOLUTION DIGITAL RADIOGRAPHY UTILIZING CCD PLANAR ARRAY", by Shaber, Lockard, and Boone, presented in December 1989, and published in SPIE, Volume 914, Medical Imaging II, page 262-269. (The present assignee provided the cooled, slow scan CCD camera and a three stage Peltier thermoelectric cooler for the CCD planar array utilized in the camera for the reported experiments. However, the apparatus and technique described fails to enable a user to reduce the x-ray dosage to the minimum levels needed to obtain the high level of spatial resolution reported and to overcome some of the disadvantages of prior radiopaque needle localization techniques.
There is a need for a technique to rapidly obtain results of x-ray mammography, lithotripsy, and bone imaging with spatial resolution and contrast resolution equal to or better than those achievable by conventional x-ray film negatives, while avoiding the time-consuming techniques and patient discomfort of prior mammography techniques and prior needle localization techniques, lithotripsy techniques, bone imaging techniques, and radiation therapy techniques.
Cooled, slow scan, low noise CCD cameras have been used in astronomy, and also have been used in various industrial radiography applications, but with much higher dosages of x-rays than is permissible in mammography and other medical applications. As used herein, the term "cooled, slow scan" refers to CCD arrays in which the CCD array is cooled, for example, by means of thermoelectric devices, to temperatures at which thermal noise is reduced to levels at which diagnostic quality radiological images can be produced. Cooled, slow scan CCD cameras presently marketed by the assignee have CCD array operating temperatures of about -20 degrees Centigrade or lower. The "slow scan" terminology refers to scanning rates that are substantially less than conventional video scan rates of 30 frames per second. The "slow scan" terminology also refers to devices in which double correlated sampling and dual slope integration techniques are electronically accomplished to minimize electronic noise. This produces improved contrast resolution and increased signal-to-noise ratios. However, it is quite problematical to determine whether such cooled, slow scan CCD cameras and techniques are practical in particular medical radiology applications such as in mammography, because medical radiology applications require minimum possible x-ray dosages, very high contrast resolution and spatial resolution. In some cases, short times for image generation are desirable, especially for needle localization techniques. Some radiological applications, for example, coronary angiography, require fast frame rates, e.g. 30 frames per second. This is inconsistent with the high signal-to-noise ratios needed to achieve the high resolutions, because fast cooled, slow scan CCD camera scanning rates needed for fast image generation result in low signal-to-noise ratios.